A hybrid electric powertrain typically includes an internal combustion engine and an electric machine. The engine usually drives the powertrain during periods of extended load. The electric machine can supplement the engine during acceleration and recovers energy during deceleration.
Some internal combustion engines include engine control systems that deactivate cylinders under low load situations. For example, an eight cylinder can be operated using four cylinders to improve fuel economy by reducing pumping losses. This process is generally referred to as displacement on demand or DOD. Operation using all of the engine cylinders is referred to as an activated mode. A deactivated mode refers to operation using less than all of the cylinders of the engine (one or more cylinders not active).
Although DOD engines improve fuel efficiency, the transition periods between activated and deactivated modes sometimes cause torque disturbances and/or decrease fuel efficiency. More particularly, as the engine transitions from the activated mode to the deactivated mode, a high frequency engine torque disturbance could be generated and may be felt by the operator as it propagates to the driveline and chassis. As the engine transitions from the deactivated mode to the activated mode, excess torque is provided as a result of the intake manifold pressure being higher than necessary. Spark retard is sometimes used to limit the excess torque as this manifold pressure ramps down, however, spark retard reduces engine operating efficiency.